The present invention relates to servo control methods and apparatus for use in a lithographic projection apparatus.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can then be imaged onto a target area (die) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent dies which are successively irradiated through the reticle, one at a time. In one type of lithographic projection apparatus, each die is irradiated by exposing the entire reticle pattern onto the die in one go; such an apparatus is commonly referred to as a waferstepper. In an alternative apparatusxe2x80x94which is commonly referred to as a step-and-scan apparatusxe2x80x94each die is irradiated by progressively scanning the reticle pattern through the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the wafer table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally  less than 1), the speed xcexd at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. In both types of apparatus, after each die has been imaged onto the wafer, the wafer table can be xe2x80x9csteppedxe2x80x9d to a new position so as to allow imaging of a subsequent die. More information with regard to lithographic devices as here described can be gleaned from International Patent Applications WO 97/33205 and WO 96/38764, for example.
Up to very recently, apparatus of this type contained a single mask table and a single substrate table. However, machines are now becoming available in which there are at least two independently movable substrate tables; see, for example, the multi-stage apparatus described in International Patent Applications WO 98/28665 and WO 98/40791. The basic operating principle behind such multi-stage apparatus is that, while a first substrate table is underneath the projection system so as to allow exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge an exposed substrate, pick up a new substrate, perform some initial alignment measurements on the new substrate, and then stand by to transfer this new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed, whence the cycle repeats itself; in this manner, it is possible to achieve a substantially increased machine throughput, which in turn improves the cost of ownership of the machine.
The projection radiation in current lithographic devices is generally UV (ultra-violet) light with a wavelength of 365 nm, 248 nm or 193 nm. However, the continual shrinkage of design rules in the semiconductor industry is leading to an increasing demand for new radiation types. Current candidates for the near future include UV light with wavelengths of 157 nm or 126 nm, as well as extreme UV light (EUV) and particle beams (e.g. electron or ion beams).
In a lithographic projection apparatus, it is necessary to control the position of the wafer table and the reticle table to an extremely high degree of accuracy, throughout multiple exposures. Vibrations in these stages may be caused inter alia by floor vibrations, indirect stepping forces, air mount noise or acoustic noise. Such vibrations reduce the image quality of the exposure; however, if operation of the system is paused so as to allow the vibrations to fade away between each exposure, this generally causes an undesirable reduction in throughput.
A particularly important characteristic of a lithographic projection apparatus is its so-called overlay precision. In a typical projection procedure, each target portion of the substrate will be subjected to various exposures, in successive irradiation sessions. These exposures will typically result in patterned layers (e.g. the circuit patterns in the various semiconductor layers of an IC) which will have to be accurately overlapped with one another. The overlay precision expresses the accuracy with which such overlap can be reproducibly achieved, and it is often of the order of nanometers. Vibrations from various sources can be transferred inter alia to the mask and substrate tables, where they can have a highly detrimental effect on the achievement of the required overlay accuracy.
It should be noted that, whilst it is relatively easy to detect the existence of vibrations as hereabove described, it requires considerable work to identify and eliminate their sources.
It is an object of the invention to address and alleviate this problem.
This and other objects are achieved according to the present invention in a lithographic projection apparatus comprising a flexible conduit carrier which connects at least a first of the movable tables to a frame outside that table.
a force sensor for measuring forces exerted on the first table by the conduit carrier and generating a force signal representative of said forces; and
a motion controller responsive to said force signal, for generating control signals to cause the positioning means connected to the first table to apply compensatory forces to that table.
The term xe2x80x9cconduit carrierxe2x80x9d as here employed refers to the xe2x80x9cumbilical cordxe2x80x9d which generally connects at least one of the movable tables to the outside frame (e.g. a metrology frame) and which carries such items as power cords, signal carriers, air tubes (e.g. for supplying air to an air bearing in the table), coolant tubes, etc. The mask table and/or the substrate table may be connected to an outside frame in this manner (using a distinct conduit carrier for each table); consequently, the term xe2x80x9cfirst tablexe2x80x9d as here employed generally encompasses both the mask table and the substrate table.
In experiments leading to the invention, the inventors performed investigations with a test lithographic apparatus, and concentrated on the achievable overlay precision with that apparatus. Although the measured precision was relatively satisfactory in the light of current standards in the semiconductor industry, it matched less satisfactorily with the standards expected to be introduced in the near future (as projection wavelengths decrease and feature sizes continue to shrink). Consequently, methods were sought to improve the attainable overlay. After much experimentation, it was eventually observed that the conduit carrier between the substrate table and the outside world was an important source of table vibration. This phenomenon was initially tackled by attempting a redesign of the conduit carrier (different materials, different dimensions, different forms, etc.), but the results of such redesigns were rather disappointing. Eventually, having realized that the phenomenon could not be alleviated in this manner, the inventors adopted an alternative approach: that of measurement and compensation instead of removal. This approach resulted in the current invention.
The present invention also provides a method of controlling the position of a movable table in a lithographic projection apparatus in which a flexible conduit carrier connects the said table to a frame outside the table, the method comprising:
measuring forces exerted on the table by the conduit carrier;
generating control signals indicative of compensatory forces to be applied to the table; and
applying compensatory forces to the table in response to said control signals, so as to counteract the effects of said forces exerted by the conduit carrier on the table.
The feedforward control provided by the present invention can provide a substantial improvement in the positioning accuracy of a movable table (e.g. a wafer table), and particularly the repeatability of positioning, by substantially compensating for the effects of vibrations caused by the presence and motion of the conduit carrier.
In a manufacturing process using a lithographic projection apparatus according to the invention, a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of energy-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget areaxe2x80x9d, respectively.